Remove extra line at the end of the file

Author: khouloudhaddad

tests/Strings/RemoveDuplicateCharactersTest.php

@@ -68,4 +68,4 @@ public function testStringWithMixedCase(): void
     {
         $this->assertSame('aAbB', removeDuplicateCharacters('aAaAaAbBbB'));
     }
-}
+}